Optical differential phase-shift-keying (DPSK) modulation is an attractive format for both high-rate fiber and free-space optical communication links. Compared to coherent phase shift keying (PSK), DPSK receivers are often easier to implement since they have relaxed linewidth requirements and can operate without optical phase locking to a local oscillator, which is a process that can extend temporal acquisition by tens of seconds and can be problematic for operation over a fading free-space channel. Compared to M-ary pulse-position modulation (M-PPM), DPSK has lower peak power, is more bandwidth efficient, and can be readily scaled to higher rates using non-adjacent differential encoding on a single channel, or using multiple wavelength division multiplexed (WDM) channels.
Currently, DPSK is usually implemented using a master oscillator power amplifier (MOPA) transmitter. A MOPA transmitter usually includes an external data modulator to generate the phase encoding. In addition, in systems using wavelength division multiplexing, each wavelength channel usually has its own external modulator. Therefore, as the number of wavelength channels increases, the size, weight, and power (SWaP) of the resulting transmitter can be too large to be implemented in SWaP-constrained applications, such as free-space communication.
Furthermore, for power-starved free-space applications, multi-rate capability can provide valuable architectural flexibility by extending the operational range of receiver (RX) power levels. This allows on-demand bandwidth when conditions are favorable, fallback modes, and operation with a variety of link conditions and transmitter (TX) and receiver (RX) designs. However, it can be challenging to scale MOPA transmitters to lower rates (e.g., about 500 Mbps or less) with good sensitivity due to limited availability of three primary elements and associated challenges: complex long-delay delay-line interferometers, matched narrow-band optical filters, and narrow-linewidth laser sources.